1. Field of the Invention
This invention relates to methods for measuring the strength and variation of gravitational fields.
2. Background of the Invention
Gravimetric measuring devices are well know in the art, although they are highly specialized and expensive systems. To make a measurement of a gravitational field, a small, well-known mass is typically employed. Because any mechanical support of the mass will also drastically effect or even obscure the relatively week effects of a gravitational field, typical means of mechanical support of a mass are not useful for gravimetrics.
For example, a typical scale to measure the weight of an item employs a spring to support the item. The mass of the item is estimated by measuring the compression of the spring when the item is suspended or supported by the spring. Variations in gravitational fields from one place to another, however, are so minute that they are immeasurable with even the most sensitive springs as the variations of the spring""s force due to temperature, mechanical vibration, etc., are many times larger than the force variations in gravitational fields.
To this end, magnetism, and more specifically repelling magnetic forces, have been employed to suspend small masses for gravimetric measurements. Magnetism can be divided into three types of magnetic behavior: diamagnetism, paramagnetism, and ferromagnetism.
Ferromagnetism is the type of magnetism most commonly employed in modern, daily life. It is the result of naturally aligned intrinsic spin axes of individual electrons of the atoms of the material. Lodestone, iron and magnetite are some of the common materials used to create xe2x80x9cpermanent magnetsxe2x80x9d, as they exhibit their strong, dipolar magnetic properties under all conditions and temperatures, with or without the presence of other types of fields.
Initially, one who is unacquainted with magnetic theories may suspect that suspension or levitation of a small mass could be achieved using an arrangement of xe2x80x9cpermanent magnetsxe2x80x9d, or ferromagnets. In 1842, however, Samuel Earnshaw proved his theorem that there is no stable configuration to levitate permanent magnets using static magnetic fields.
Some quasi-stable levitation arrangements have been achieved by spinning the levitated mass, in which gyroscopic moments offset the inherent instability of the forces otherwise exerted on the suspended mass. Unfortunately, the gyroscopic forces are also large enough to offset or obscure the effects of small forces, such as variations in gravitational field, on the suspended mass. Additionally, energy must be induced into the spinning mass to keep it spinning over time, which may also obscure gravimetric measurements.
Diamagnetism may be viewed as an atomic version of Lenz""s Law which provides that an electric current resulting from an applied magnetic field will be in a direction which sets up an opposing magnetic field. For example, if a dipole rod magnet (31) is passed or moved v(t) through an electrically conductive ring (30), a current i(t) will be induced in the ring in a direction which sets up a magnetic field (32, 32xe2x80x2) which opposes the movement v(t) of the rod magnet, as shown in FIG. 1. This reactive current and opposing magnetic field is created regardless of the polarity of the inducing magnet.
All known elements are believed to exhibit some degree of diamagnetism. Most elements, however, do not exhibit noticeable or measurable diamagnetic properties. Under cryogenic conditions, such as 77 K, superconductive properties of many elements allow for substantial diamagnetic properties.
Under such cryogenic conditions, Lenz""s law can be applied to statically levitate a small magnetic mass above a strong diamagnetic material, taking advantage of the Meisner Effect in which movements of the levitated mass result in a reactive and opposing field to correct for the movements, thereby leaving it in a stable position suspended above the diamagnetic material without any means of mechanical support. For example, as shown in FIG. 2, a small magnetic mass (41), such as a Samarium Cobalt magnet, can be levitated a distance d above a superconductive diamagnetic material (42) such as a ceramic Yttrium compound, which is superconductive at temperatures such as 77 K.
Such a cryogenic, diamagnetic arrangement has been employed by some gravimetric measurement systems, as the force between the levitated mass and the diamagnetic base is highly stable and constant, thereby allowing any differences in displacement between the base and the levitated mass to be attributed to the tiny variations in gravitational field. A laser interferometer may be employed to accurately measure the position of the levitated mass.
This type of gravimetric arrangement, however, is highly dependent on maintaining cryogenic conditions, which implies a need for a considerable supply of coolant such as liquid Nitrogen. Additionally, this type of system is difficult and expensive to operate due to the cryogenesis. Therefore, there is a need in the art for a system and method for gravimetric measurement which avoids the need for superconductive conditions, materials, and supplies.